The present invention relates to a method for making masks used in photolithography for making semiconductors. In particular, the present invention relates to a method for making such a mask that is unaffected by substrate deformation.
When making a photolithographic mask, absorption film 101 and photoresist layer 102 may be deposited on substrate 100, as shown in FIG. 1a. The stresses those layers apply to substrate 100 can deform it by bending it. That deformation could cause displacement errors that render the resulting mask unsuitable for generating sub 0.1 micron features.
FIGS. 1b-1e illustrate how such deformation can produce unacceptable displacement errors. After film 101 and layer 102 are formed, an e-beam writing step is performed to define those portions of photoresist layer 102 that will be removed prior to etching absorption film 101. That e-beam writing step is thus applied to a deformed substrate. The resulting structure is shown in FIG. 1b. The photoresist is then developed, generating the structure shown in FIG. 1c. Removing part of photoresist layer 102 releases some of the stress, causing substrate 100 to deform relative to its shape during the e-beam writing step. In essence, the substrate starts to xe2x80x9csnap backxe2x80x9d from the bent position it assumed during that step.
Next, absorption film 101 is etched (FIG. 1d) and the remaining portions of photoresist layer 102 are removed (FIG. 1e). These steps release additional stress, causing substrate 100 to further deform relative to its shape during the e-beam writing step. By applying the e-beam writing step to a bent substrate, which subsequently snaps back toward its initial shape (i.e., its shape prior to the deposition of film 101 and layer 102), the resulting mask defines features that are displaced from where they had been prescribed by the e-beam writing step.
FIG. 2 provides a graphical representation for how this process can cause displacement errors. Depositing film 101 and layer 102 (steps 201 and 202, respectively) applies stresses that cause in plane distortion 210 and 211, respectively. After the e-beam writing step (step 203), photoresist removal, absorption film etching, and photoresist stripping steps (steps 204, 205, and 206, respectively) release stress, which causes substrate 100 to return toward its initial shape. This can cause features, demarcated by the e-beam writing step, to be displaced relative to features that the resulting mask defines.
Accordingly, there is a need for a method for making a photolithographic mask that reduces the deformation caused by depositing films on a substrate. The present invention provides such a method.